This invention relates to a process for forming color images using photosensitive silver halide materials.
Developing solution generally suffers from some problems that a very careful preparation is required because it contains reducing agents such as a developing agent and hydroxylamine in alkaline water, and a great volume of developing solution cannot be prepared at a time because it is liable to changes during shelf storage.
One solution is to incorporate a reducing agent such as a developing agent in a photosensitive material and treat the material with an alkaline bath generally known as activator bath. This approach, which in turn, undesirably deteriorates the shelf stability of the photosensitive material prior to the development, has not been commercially successful as a color image forming process.
For the purpose of overcoming these problems, it is desired to reduce the pH of developing solution as low as possible. However, low pH levels naturally bring out another problem of taking a long developing time.
U.S. Pat. No. 3,260,598 discloses an image forming process utilizing the mechanism wherein alkali-releasing agents, a very slightly water-soluble metal hydroxide and a compound XY are reacted to release hydroxyl ions to increase the pH wherein X represents a sodium or potassium ion, and Y represents a citrate ion, an oxalate ion, a fluoride ion, a ferricyanide ion, a tartrate ion, a sulfite ion, an ethylenedinitrilo tetraacetate ion, a 1,3-diamino-2-propanol tetraacetate ion, a trimethylamine triacetate ion, and other aliphatic nitrogenous polycarboxylate ions.
This patent discloses that the process finds predominant applications in black and white images forming processes, typically silver salt diffusion transfer process, but is silent about technical considerations required when the process is applied to color image forming processes. More specifically, insofar as our empirical runs are concerned, the treating process of the above-referred patent, which is successful in lowering the pH of developing solution, fails to accomplish a sufficient image density when conventional well known four equivalent couplers are used.